Optical coherence tomography laser with integrated clock

ABSTRACT

A frequency swept laser source for TEFD-OCT imaging includes an integrated clock subsystem on the optical bench with the laser source. The clock subsystem generates frequency clock signals as the optical signal is tuned over the scan band. Preferably the laser source further includes a cavity extender in its optical cavity between a tunable filter and gain medium to increase an optical distance between the tunable filter and the gain medium in order to control the location of laser intensity pattern noise. The laser also includes a fiber stub that allows for control over the cavity length while also controlling birefringence in the cavity.

RELATED APPLICATIONS

This application is a Divisional of U.S. application Ser. No.12/396,099, filed on Mar. 2, 2009, which claims the benefit under 35U.S.C. 119(e) of U.S. Provisional Application No. 61/053,241, filed onMay 15, 2008, both of which are incorporated herein by reference intheir entirety.

BACKGROUND OF THE INVENTION

Optical coherence analysis relies on the use of the interferencephenomena between a reference wave and an experimental wave or betweentwo parts of an experimental wave to measure distances and thicknesses,and calculate indices of refraction of a sample. Optical CoherenceTomography (OCT) is one example technology that is used to performusually high-resolution cross sectional imaging. It is often applied toimaging biological tissue structures, for example, on microscopic scalesin real time. Optical waves are sent through an object or sample and acomputer produces images of cross sections of the object by usinginformation on how the waves are changed.

The original OCT imaging technique was time-domain OCT (TD-OCT), whichused a movable reference mirror in a Michelson interferometerarrangement. Another type of optical coherence analysis is termedFourier domain OCT (FD-OCT). Other related OCT techniques are timeencoded and spectrum encoded Frequency Domain OCT. These Fourier domaintechniques use either a wavelength swept source and a single detector,sometimes referred to as time-encoded FD-OCT or TEFD-OCT, or a broadbandsource and spectrally resolving detector system, sometimes referred tospectrum-encoded FD-OCT or SEFD-OCT. FD-OCT has advantages over timedomain OCT (TD-OCT) in speed and signal-to-noise ratio (SNR).

TEFD-OCT has advantages over SEFD-OCT in some respects. The spectralcomponents are not encoded by spatial separation, but they are encodedin time. The spectrum is either filtered or generated in successivefrequency steps and reconstructed before Fourier-transformation. Usingthe frequency scanning light source (i.e. wavelength tuned laser) theoptical configuration becomes less complex but the critical performancecharacteristics now reside in the wavelength tuned laser and especiallyits tuning speed and accuracy.

SUMMARY OF THE INVENTION

Frequency swept laser source for TEFD-OCT imaging requires tuning atvery high repetition rates, in the tens of kilohertz, for fast real-timeimage frame acquisition with a sufficiently large image pixel count. Atthe same time, long coherence length of the source is required for largeimaging depth ranges.

For high resolution/low noise imaging, swept laser OCT systems requireeither an extremely linear optical frequency sweep in time, or somemechanism to measure and compensate for nonlinear tuning.

Many OCT systems use a stand alone Fabry-Perot, Mach-Zehnder or someother type of fixed interferometer to provide frequency markers equallyspaced in optical frequency to trigger digitization of the signal fromthe sample as the swept laser is tuned through its frequency scan band.These implementations are often done with fiber-optic components. Theproblem with these methods is cost in assembly labor, cost incomponents, and physical size.

In general, according to one aspect, the invention features a clocklaser system comprising an optical bench, a tunable laser source, on theoptical bench, that generates an optical signal that is tuned over aspectral scan band, and a clock subsystem on the optical bench thatgenerates clock signals as the optical signal is tuned over the scanband.

In operation, a portion of the optical signal generated by the tunablelaser is directed to the clock subsystem.

In the preferred embodiment, the tunable laser source comprises a frontreflector, through which the optical signal is provided, and a backreflector, through which light generated in the tunable laser source isprovided to the clock subsystem, wherein the front reflector and theback reflector define a laser cavity of the tunable laser source.

At least one lens component is typically used on the optical bench forcollimating the optical signal from the tunable laser source andreceived by the clock subsystem.

In the current embodiment, the clock subsystem comprises an etalon forspectrally filtering the tunable signal, a detector for detecting theoptical signal filtered by the etalon to produce the clock signals, anda beam splitter for directing a portion of the optical signal to theetalon and directing the filtered optical signal returning from theetalon to the detector.

In general, according to another aspect, the invention is characterizedas a method for generating a clock signal for a tunable laser. Themethod comprises generating a tunable signal that is scanned over aspectral scan band in a tunable laser that is implemented at least inpart on an optical bench, filtering a portion of light generated in thetunable signal to a filter installed on the optical bench, andgenerating clock signals from the light filtered by the filter with adetector installed on the bench. These clock signals indicate scanningof the tunable signal through fixed frequency increments.

Another problem that can arise in tunable lasers, and when tunablelasers are used in optical coherence systems is noise from the lasers.Laser pattern noise can arise from parasitic reflections within thesystem and particularly the laser.

In general, according to another aspect, the invention features anoptical coherence analysis system. This system comprises aninterferometer for using an optical signal to analyze a sample and atunable laser for generating the optical signal. The tunable laserincludes a gain medium for amplifying optical signals within a lasercavity of the tunable laser, a tunable filter for tuning the opticalsignals over a spectral scan band, and a cavity extender in the opticalcavity to increase an optical distance within the cavity. This extenderhas the effect of moving laser pattern noise to a region where it isless problematic for the optical coherence analysis system.

In the current embodiment, the cavity extender is located between thetunable filter and the gain medium to increase an optical distancewithin the laser cavity, more specifically, extending an opticaldistance between a lens component, which couples light between thetunable filter and the gain medium, and the tunable filter. Also, thecavity extender comprises a transparent substrate that is antireflectioncoated. Example materials include silica and silicon.

In general according to another aspect, the invention is characterizedas a method for producing a tunable optical signal for an opticalcoherence analysis with reduced laser pattern noise. This methodcomprises generating the tunable optical signal in a laser cavity thatcomprises a train of optical elements: a gain medium, an intracavitytuning element, and at least one lens component for coupling lightbetween the gain medium and the intracavity tuning element. The tunableoptical signal is directed to an optical coherence analysis system. Toaddress noise, a transmissive high refractive index component isinserted in the laser cavity to increase an optical distance betweenoptical elements that produce spurious reflections.

In general according to another aspect, the invention features asampling clock system for an optical coherence analysis system,comprising: an optical detector for detecting an optical clock signalgenerated by optically filtering a tunable signal from a tunable laserthat generates light for the optical coherence analysis system and anelectronic time delay circuit for delaying the signal from the opticaldetector by an amount corresponding to the propagation time of the lightfrom the tunable laser in the optical coherence analysis system.

In the preferred embodiment, the electronic time delay circuit has aprogrammable delay. A high pass filter is also preferably provided forfiltering the signal from the optical detector. Further, a frequencydivider or multiplier is useful sometimes for decreasing or increasing afrequency of the signal from the optical detector.

Tunable lasers based on semiconductor gain media typically producehighly polarized light. It is further desirable to have a laser with ahybrid free space/fiber cavity, since the cavity length can be easilyadapted to different optical coherence applications withoutnecessitating a redesign. However, the use of the fiber can impact thetuning performance of the laser since it can introduce birefringence.

In general according to another aspect, the invention features a tunablelaser for generating frequency tunable optical signals. The tunablelaser includes a gain medium for amplifying optical signals within alaser cavity of the tunable laser, a tunable filter for tuning theoptical signals over a spectral scan band, a fiber stub within the lasercavity between the gain medium and an end reflector of the laser cavity,and a fiber birefringence controller for changing a birefringencecharacteristics of the fiber stub. This is used to calibrate the laserto have the desired tuning characteristics.

Currently, the birefringence controller applies mechanical stress to theoptical fiber by twisting the fiber.

In general according to another aspect, the invention is characterizedas a method for calibrating an external cavity tunable laser having ahybrid free space and fiber cavity. The method comprises generatingoptical signals within a laser cavity of the tunable laser, tuning theoptical signals over a spectral scan band, transmitting the opticalsignals between an end reflector of the laser cavity and the gain mediumin an optical stub, changing the stress applied to the optical fiberuntil desired tuning characteristics are observed in the optical signal,and fixing the stress applied to the optical fiber.

Preferably, the stress applied to the optical fiber is changed to reducepolarization fading over the scan band of the tunable signal, to reducechanges in power as a function of frequency in the optical signal duringtuning of the frequency of the optical signal, and/or to linearizechanges in frequency as a function of time during tuning of the opticalsignal.

The above and other features of the invention including various noveldetails of construction and combinations of parts, and other advantages,will now be more particularly described with reference to theaccompanying drawings and pointed out in the claims. It will beunderstood that the particular method and device embodying the inventionare shown by way of illustration and not as a limitation of theinvention. The principles and features of this invention may be employedin various and numerous embodiments without departing from the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, reference characters refer to the sameparts throughout the different views. The drawings are not necessarilyto scale; emphasis has instead been placed upon illustrating theprinciples of the invention. Of the drawings:

FIG. 1 is a scale plan view of a tunable laser with integrated clock;

FIG. 2 is a plot of transmission and reflection for the clock etalon;

FIG. 3 is a schematic block diagram of an optical coherence analysissystem;

FIG. 4A is a plot of laser intensity pattern noise as function ofoptical path lengths within the sample showing a spurious noise peaklocated at about 5.1 millimeters (mm) sample depth;

FIG. 4B is a plot of laser intensity pattern noise as function ofoptical path lengths within the sample showing a spurious noise peakmoved to about 7.8 mm sample depth by operation of an optical pathlength extender element inside the laser cavity;

FIG. 4C is a plot of laser intensity pattern noise as function ofoptical path lengths within the sample showing a spurious noise peakmoved beyond 10 mm sample depth by operation of an optical path lengthextender element inside the laser cavity; and

FIG. 5 is a plan view of an optical coupler providing intracavitybirefringence control for the tunable laser.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an integrated laser clock system 100 that has beenconstructed according to the principals of the present invention.

Generally the integrated laser clock system 100 comprises a tunablelaser subsystem 50, which generates a wavelength or frequency tunableoptical signal, and a clock subsystem 200, which generates clock signalsat spaced frequency increments as the tunable signals or emissions ofthe laser 50 are spectrally tuned over a spectral scan band. The clocksignals are used to trigger sampling, typically in an OCT system.

The tunable laser subsystem 50 and clock subsystem 200 of the integratedlaser clock system 100 are integrated together on a common optical bench110. This bench is termed a micro-optical bench and is preferably lessthan 10 millimeters (mm) by 20 mm in size so that it fits within astandard butterfly or DIP (dual inline pin) hermetic package 101. In oneimplementation, the bench is fabricated from aluminum nitride. Athermoelectric cooler 102 is disposed between the bench 110 and thepackage 101 (attached/solder bonded both to the backside of the benchand inner bottom panel of the package 101) to control the temperature ofthe bench 110.

In more detail, the tunable laser 50 in the preferred embodiment inbased on the tunable laser designs disclosed in U.S. Pat. No. 7,415,049B2, which is incorporated herein in its entirety by this reference.

In the current implementation, the tunable laser 50 comprises asemiconductor gain chip 52 that is paired with amicro-electro-mechanical (MEMS) angled reflective Fabry-Perot tunablefilter 54 to create external cavity laser (ECL) with the tunable filter54 being an intracavity tuning element and forming one end, or backreflector, of a laser cavity 56 of the tunable ECL 50.

The semiconductor optical amplifier (SOA) chip 52 is located within thelaser cavity 56. In the current embodiment, both facets of the SOA chip52 are angled relative to a ridge waveguide 58 running longitudinallyalong the chip 52 and anti-reflection (AR) coated, providing parallelbeams from the two facets. The SOA chip 52 is mounted on a submount 66that, in turn, is mounted on the top side of the optical bench 110,typically by solder bonding.

To collect and collimate the light exiting from each end facet of theSOA 52, two lens structures 60, 62 are used. Each lens structure 60, 62comprises a LIGA mounting structure m, which is deformable to enablepost installation alignment, and a transmissive substrate s in which thelens is formed. The transmissive substrate s is typically solder orthermocompression bonded to the mounting structure m, which in turn issolder bonded to the optical bench 110.

The first lens component 60 couples the light between the back facet ofthe SOA 52 and the tunable filter 54. Light exiting out the front facetof the SOA 52 is coupled by a second lens component 62 to optical fiberstub 64 via its front facet. The optical fiber stub 64 is alsopreferably solder attached to the optical bench 110 via a mountingstructure m. The output optical fiber 64 is usually single spatial modefiber (SMF).

The optical fiber 64 transports the tunable signal to a coupler 65. Herethe optical fiber terminates in a reflective coating 68, for a frontreflector of the laser cavity 56, having between 1 and 85 percentreflectivity to feedback a portion of the tunable signal into the lasercavity 56. The light transmitted through the reflective coating 68 istransported to the optical coherence analysis system 300, see FIG. 3,via fiber pigtail 320.

The use of the fiber stub 64 in the cavity yield an ECL that has hybridfree space and fiber cavity, with the free space portion beingimplemented on the optical bench. The use of the stub 64 provides forcontrol over the length of the laser cavity 56 by adjusting the lengthof the stub without necessitating changes to the portion of the opticalcavity implemented on the bench 110. Generally, the cavity length 56 ofthe laser is selected in dependence upon the desired tuning speed: toolong a cavity/fiber stub 64 may prevent the laser from lasing due to thetransit time of light from the amplifier to the fiber exceeding thetunable filter bandwidth dwell time during a scan; but too short a fibercavity will contain too few longitudinal modes in the laser emissiondefined by the linewidth of the tunable filter and contribute to highrelative intensity noise (RIN) and tuning instability. In the currentembodiment, for scan speeds ranging from 10 kHz to 100 kHz, fiberlengths vary from 30 centimeters (cm) to 3 cm.

The angled reflective Fabry-Perot filter 54 is a multi-spatial-modetunable filter having a curved-flat optical resonant cavity thatprovides angular-dependent, reflective spectral response back into thelaser cavity 56. This effect is discussed in more detail in incorporatedU.S. Pat. No. 7,415,049 B2. In the referred embodiment, the curvedmirror is on the MEMS membrane and is on the side of the filter 54 thatadjoins the laser cavity 56. The flat mirror is on the opposite side andfaces the laser cavity 56. The flat mirror preferably has a higherreflectivity than the curved mirror. Currently the reflectivities forthe flat and curved mirrors are typically 99.98% and 99.91%,respectively, in order to achieve the desired reflectivity and requisitelinewidth of the filter 54 in reflection.

The light transmitted by the tunable filter 54 is coupled out of thelaser cavity 56 and into the clock subsystem 200 to be collimated by athird lens component 210 and a fourth lens component 212, which aresolder bonded to the optical bench 110. Two fold mirrors 214 and 216,which are reflective coated substrates that are solder bonded to thebench 110, fold the beam of the light from the tunable laser subsystem50, allowing for a dimensionally compact system.

The light then passes through a beam splitter 222, which is preferably a50/50 splitter to a clock etalon 220. Any light reflected by thesplitter 222 is directed to a beam dump component 224 that absorbs thelight and prevents parasitic reflections in the hermetic package 101 andinto the laser cavity 56.

The clock etalon 220 functions as a spectral filter. Its spectralfeatures are periodic in frequency and spaced spectrally by a frequencyincrement related to the refractive index of the constituent material ofthe clock etalon 220, which is fused silica in one example, and thephysical length L of the clock etalon 220. The etalon can alternativelybe made of other high-index and transmissive materials such as siliconfor compactness, but the optical dispersion of the material may need tobe compensated for with additional processing inside the DSP. Also,air-gap etalons, which are nearly dispersionless, are anotheralternative.

The contrast of the spectral features of the etalon is determined by thereflectivity of its opposed endfaces 220 a, 220 b. In one example,reflectivity at the etalon endfaces 220 a, 220 b is provided by theindex of refraction discontinuity between the constituent material ofthe etalon and the surrounding gas or vacuum. In other examples, theopposed endfaces 220 a, 220 b are coated with metal or preferablydielectric stack mirrors to provide higher reflectivity and thuscontrast to the periodic spectral features.

In the illustrated example, the clock etalon 220 is operated inreflection. FIG. 2 is a plot of reflection/transmission as a function ofoptical frequency in units of the free spectral range (FSR) of theetalon 220. Thus, as the light from the laser scans through reflectionpeaks and troughs located at the fixed frequency increments, a clocksignal is formed indicating each time the laser has scanned throughanother frequency increment.

The FSR of the clock etalon is chosen based on the required scanningdepth in an OCT system. The Nyquist criterion dictates that the periodicfrequency spacing of the clock etalon that defines the sample rate behalf the smallest frequency period component of the sample, thus settingthe optical thickness of the clock etalon to twice the required imagingdepth. However, as is typically done with clock oscillators, theperiodic waveform can be electrically frequency doubled, tripled, etc,or can be halved to obtain the desired sample rate while choosing anetalon of a length that is convenient for handling and that easily fitswithin the package 101 and on the bench 110. A thicker etaloncompensates better for nonlinear frequency scanning than a thinner onedue to its finer sample rate, but it is larger and more difficult tofabricate, so a tradeoff is made depending upon the laser tuninglinearity, system depth requirements, and manufacturing tolerances.Moreover, a thicker etalon requires a laser of comparable coherencelength to generate stable clock pulses, so the laser coherence lengthcan also help dictate the design of the etalon thickness.

Returning to FIG. 1, the light returning from the clock etalon 220 andreflected by beamsplitter 222 is detected by detector 226. The lightdetected by detector 226 is characterized by drops and rises in power asthe frequency of the tunable signal scans through the reflectivetroughs/reflective peaks provided by the clock etalon 220.

The detector photocurrent is amplified with a transimpedance amplifier310. Its signal is sent through a high-pass filter 312 to remove thedirect current (DC) and other low-frequency components. The object ofthis processing is to provide an oscillating signal centered about zeroVolts where the zero-crossings can be detected to generate a preciseclock signal. Further shaping of the clock is provided by an amplifier314, which can be a linear type, an automatic gain control type, or alimiting type amplifier. The amplifier 314 is followed by an optionalfast comparator 316 to detect zero crossings and convert the analogsignal to a clean digital clock signal. The clock signal is multipliedor divided in frequency by multiplier/divider 318, depending on theneeds of the OCT system's application and the requirement for aconvenient etalon (or other clock interferometer) size within thebutterfly package 101.

In one embodiment, a frequency multiplier 318 is used to increase thesampling frequency to the multiple of the signal from the clockingdetector. This is used in situations in which a higher samplingfrequency is required. One case where it is advantageous is forapplications that require a very deep scan range. For example, if a scanrange of L is required, the etalon length would have to be at least 2L/n, where n is the refractive index. By installing frequency doublingelectronics, the etalon length would only have to be L/n. This may makethe difference between an etalon that fits inside the package and not.Another reason for using multiplication electronics is that it canincrease stability of the clock signal for a given laser coherencelength. The interference fringes are more stable for shorter etalonlengths. It is typically the case that clocks from a shorter etalon, butlater frequency multiplied, are more stable overall. Alternatively,using a shorter clocking etalon with electronic frequency multiplicationallows using a laser with a smaller coherence length while maintaininggood clock stability.

Light that is transmitted through the etalon is absorbed by a secondbeam dump component 225 that absorbs the light and prevents parasiticreflections into the laser cavity. The transmission signal throughetalon 220 can alternatively be used as a marker. In thisimplementation, the detector is placed in the position of the beam dump225. Currently, the reflection signal is preferred because of its low DCoffset (higher contrast signal).

FIG. 3 shows an example of how the laser/clock module 100 of FIG. 1 isused in one, exemplary optical coherence analysis system 300, and morespecifically the Michelson interferometer that is used to analyze theoptical signals from the sample. Many other OCT system interferometerconfigurations and sample probe optics can be used with the laser/clockmodule 100, however.

In more detail, the illustrated system is a time encoded Fourier domainoptical coherence tomography system (TEFD-OCT). The light from thetunable laser 50 is output on fiber 320 to a 90/10 optical coupler 322.The tunable signal is divided by the coupler 322 between a reference arm326 and a sample arm 324 of the system.

The optical fiber of the reference arm 326 terminates at the fiberendface 328. The light exiting from the reference arm fiber endface 328is collimated by a lens 330 and then reflected on a mirror 332 to returnback. It is then directed by circulator 342 to 50/50 fiber coupler 346.

The external mirror 332 has an adjustable fiber to mirror distance (seearrow 334). This distance determines the depth range being imaged, i.e.the position in the sample 340 of the zero path length differencebetween the reference arm 326 and the sample arm 324. The distance isadjusted for different sampling probes and/or imaged samples. Lightreturning from the reference mirror 332 is returned to a reference armcirculator 342 and directed to the 50/50 fiber coupler 346.

The fiber on the sample arm 324 terminates at the sample arm probe 336.The exiting light is focused by the probe 336 onto the sample 340. Lightreturning from the sample 340 is returned to a sample arm circulator 341and directed to the 50/50 fiber coupler 346. The reference arm signaland the sample arm signal are combined in the fiber coupler 346. Thecombined signal is detected by a balanced receiver, comprising twodetectors 348, at each of the outputs of the fiber coupler 346.

In examples, the scanning is implemented by moving the probe 336relative to the sample 340 using a two (x-y) dimensional or three(x-y-z) dimensional positioner 337. In other examples, the x-y-zscanning is implemented by moving the sample 340 relative to the probe336. In still other examples, cylindrical scanning is implemented byrotating and axially moving the probe 336.

The clock signal is produced by the clock subsystem 200 of the clocklaser 100, and is further formed and conditioned by the transimpedanceamplifier 310, high-pass filter 312, amplifier 314, optional fastcomparator 316, and optional multiplier/divider 318 as describedpreviously.

A programmable time delay circuit 319 is also provided. This delays thesampling/clock signal by a period determined by a user or electronicallyby DSP 380. The delay is selected so that the clock signal is aligned intime with the optical signals detected by the balanced detectors 348 ofthe OCT system and amplified by amplifier 350. Specifically the timedelay delays the clock signals for a time corresponding to the delayassociated with the optical signals propagation through the opticalfibers in the interferometer arms.

The main interferometer in an OCT system typically has long lengths offiber in it, causing long time delays. The clock, however, does notsuffer these delays because it is very close to the laser. It isessential that delay mismatch be minimized for highest OCT systemperformance, especially in terms of resolution. Since the delay mismatchcannot be made up optically in a compact system, an electronic delayline is useful. The electronic delay is set to match the delay of theOCT system and is programmable in a system designed for flexibility.

An analog to digital converter system 315 is used to sample the outputfrom the amplifier 350. The clock input from the clock subsystemprovides time of the sampling at equally spaced swept optical frequencyincrements.

Once a complete data set has been collected from the sample 340 by theoperation of the scanner and the spectral response at each one of thesepoints is generated from the tuning of the laser-laser clock module 100,the digital signal processor 380 performs a Fourier transform on thedata in order to reconstruct the image and perform a 2D or 3Dtomographic reconstruction of the sample 340. This information generatedby the digital signal processor 380 is then displayed on a videomonitor.

Cavity Extender

Parasitic reflections in the laser cavity give rise to laser patternintensity noise, or laser intensity variation as the laser is tuned.This manifests itself as artifacts in the reconstructed images,appearing as spurious features at intra-sample positions correspondingto the laser intracavity distances between parasitic reflections. In theembodiment illustrated in FIG. 1, a major contributor to these artifactsis the parasitic reflections between tunable filter 54 and the lensstructure 60. The distance between these two reflectors, ICR, give riseto laser pattern noise at the equivalent optical distance with thesample 340. This is especially problematic when the optical distance ICRcorresponds to optical distance of interest within the sample 340.

FIG. 4A is a plot of laser pattern noise as a function of opticaldistance or depth, for example within the sample. Here optical distancemeans the equivalent distance in an air or vacuum. Thus thecorresponding physical distance is scaled by the refractive index of thetransmission media, such as the sample. Artifact generated by peak 414corresponding to the reflection between the tunable filter 54 and lensstructure 60 is problematic if the sample distance overlaps this peak.One way to solve this potential problem is to reduce the magnitude ofthe spurious reflections and thus the magnitude of the spurious peak414. However, sufficient reduction of the peak magnitude is not alwayspossible. Another possible approach is to move this spurious peak to anoptical distance location which is outside of the imaging range ofinterest. In this case the peak either does not appear at all inside theframe of the image, which can excise the undesired depth region, or, ifthe peak is folded into the image by aliasing from signal sampling, thepeak can be sufficiently attenuated by the sampling anti-aliasing filterin the system. Moving the spurious peak to a different location requireschanging optical distance between the two components producing theinterference of their reflections. Sometime moving these componentpositions might be sufficient. However, the filter position might befixed in order for the lens to mode-match the beam size from thesemiconductor optical amplifier to the filter mode size using the lens.What is desired is a method to maintain mode-matching while extendingthe optical path length between the filter and the lens.

FIG. 1 shows a tunable laser with an optical path length extenderelement 70 inside the laser cavity 56. In the preferred embodiment, theextender element 70 is a transparent high refractive index material,such as fused silica or silicon or other transmissive material having arefractive index of about 2 or higher. Currently silicon is used. Bothendfaces 72, 74 of the extender element 70 are antireflection coated.Further, the element is preferably angled by between 1 and 10 degreesrelative to the optical axis of the tunable laser 50 to further spoilany reflections from the its endfaces 72, 74 from entering into thelaser beam optical axis. This extender element is placed between the twointerfering reflectors 54 and 60 that cause the spurious peak 414, andthe purpose of this extender element 70 is to change the opticaldistance between the two reflectors and thus change the length positionof the spurious peak while not necessarily necessitating a change in thephysical distance between the elements.

Since silicon has a refractive index of about 3.5 for relevant opticalwavelengths, the extender element 70 has the effect of moving the noisepeak 414 in FIG. 4A to the right to correspond to greater depths withinthe sample 340 so that the laser pattern noise is now outside the depthof interest within the sample 340.

FIG. 4B is a plot of laser intensity pattern noise as a function ofoptical path lengths with the new position of the spurious noise peak at414 b due to the operation of the extender 70. Furthermore, the highindex of the silicon reduces the beam divergence and thus permits thefilter to be moved farther away from the lens and still remainmode-matched.

Alternatively, the extender element 70 can be chosen to place the laserpattern beyond the imaging depth of the system such that the patternfolds over into the image, but the anti-aliasing filter reduces thepattern magnitude to a negligible level, as shown in FIG. 4C.

In a current embodiment, the distance ICR is about 10 millimeters andthe length of the extender element is about 2 millimeters.

Intracavity Birefringence Control

Typically, the semiconductor optical amplifier (SOA) 52 has gain in onlyone preferential polarization direction. As described previously, lightreflected from the fiber mirror coating 68 in the coupler 65 serves asone mirror of the laser cavity. However, single-mode fiber (nonpolarization-maintaining), which is often preferred for OCT applications(polarization maintaining (PM) fiber can cause ghost images due to somelight coupling into the undesired fiber axis and experiencing a longertime delay), can arbitrarily rotate the polarization state of the light(called birefringence) and thus degrade laser feedback over the tuningrange. Further, stress from soldering the fiber further inducesbirefringence. In the worst-case instance, it can prevent lasing if thereflected light's direction of polarization ends up orthogonal to thepreferred polarization state of the optical amplifier 52.

Inserting a fiber polarization controller in the laser cavity 56compensates by properly aligning the polarization state of the reflectedlight, but requires a long length of fiber and thus will not work withshort fiber cavity lasers that are required for higher speed tuning Whatis desired for a short fiber cavity (from 3 to 8 cm) is a device thatallows for polarization alignment and fiber stability in a compactfixture, as shown in FIG. 5 and described below.

The fiber stub 64 is held by two points in the coupler 65: 1) solder atpoint 418 connects fiber 64 to tube 420 that is the coupler's ferrule;and 2) by the mechanical splice between fiber 64 and fiber 320 at theoptical coating/laser end reflector 68.

One example of a mechanical splice is the 3M™ Fibrlok™ II UniversalOptical Fiber Splice, which is mechanical splice that allows thecoupling the highly reflective (HR) coated fiber 64 to the output fiber320. In other examples, a fusion splice with an HR coating is used tocouple fiber 64 to fiber 320.

This mechanical splice is fixed within the body 422 typically by anepoxy bond. A cylindrical holder 424 is fixed to the body 422 and has ininner bore into which the tube 420 is inserted. A set screw 426 enablesthe tube 420 to be rotated relative to the cylindrical holder 424 andthen fixed to the holder 424 when the set screw 426 is tightened down.This allows a mechanical stress to be imparted to fiber 64 in its lengthbetween solder 418 and the region of the splice 68 that is secured tobody 422. This stress affects the birefringence of the fiber 64 and thusaffects the polarization state of the light within the laser cavity 56.

In operation and typically in a manufacturing/calibration stage, withthe set screw 426 loosened, an operator twists the fiber 64 by rotatingthe body 422 about the stainless steel tube 420. The output of thetunable laser operation is monitored by a detector connected to a highspeed oscilloscope. Twisting of the fiber stub 64 induces polarizationchanges. When optimal operation is observed, characterized by a maximumpower output from the laser 100 and smooth tuning, i.e., reduced changesin power as a function of frequency during tuning and the change infrequency as a function of time is linear or near linear during tuning,over the desired tuning range without any polarization fading, the setscrew 426 is tightened to fix the stress applied to the fiber. Thestainless steel tube 420 prevents any subsequent fiber movement and thusmaintains polarization stability.

Fiber 64 is single mode fiber. Because of bends in it and possiblystress at various points along its length, such as soldering points onthe bench LIGA m and tube ferrule 420, has some small amount ofresulting birefringence. This means that the light sent into the fiberfrom the gain chip 52 and bench 110 does not come back from thefiber-end mirror 68 necessarily in the same polarization as it waslaunched. By fixing the fiber 64 in this coupler 65, and rotatingslightly the fiber, the polarization of the light returning from themirror 68 is matched to the launch polarization. This polarizationmatching occurs over the whole wavelength tuning range of the laser 100.Any residual birefringence of the SMF fiber is small.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. An optical coherence analysis system, comprisingan interferometer for using an optical signal to analyze a sample; and atunable laser for generating the optical signal, the tunable laserincluding: a gain medium for amplifying the optical signal within alaser cavity of the tunable laser; a tunable filter for tuning theoptical signal over a spectral scan band and located in a free spaceportion of the laser cavity; and a cavity extender, comprising atransmissive substrate, in the free space portion of the optical cavityto increase an optical distance within the cavity, the cavity extenderbeing angled relative to an optical axis of the tunable laser, whereincavity extender is spaced away from the tunable filter.
 2. A system asclaimed in claim 1, wherein the cavity extender is located between thetunable filter and the gain medium to increase an optical distancebetween them in the free space portion of the optical cavity.
 3. Asystem as claimed in claim 1, wherein the gain medium is a semiconductorgain medium.
 4. A system as claimed in claim 1, wherein the tunablefilter comprises a Fabry-Perot tunable filter.
 5. A system as claimed inclaim 1, wherein the tunable filter comprises a MEMS Fabry-Perot tunablefilter.
 6. A system as claimed in claim 1, wherein the tunable filtercomprises a MEMS angled reflective Fabry-Perot tunable filter.
 7. Asystem as claimed in claim 1, wherein the cavity extender comprises atransparent substrate that is antireflection coated to suppressreflections at endfaces of the transparent substrate within the freespace portion of the laser cavity.
 8. A system as claimed in claim 1,wherein the cavity extender comprises a rectangular transparentsubstrate that is angled relative to an optical axis of the laser cavityto suppress reflections at endfaces of the transparent substrate withinthe free space portion of the laser cavity along the optical axis.
 9. Asystem as claimed in claim 1, wherein the cavity extender extends anoptical distance between a lens component, which couples light betweenthe tunable filter and the gain medium, and the tunable filter, all ofwhich are located within the free space portion of the laser cavity. 10.A system as claimed in claim 1, wherein the cavity extender is formedfrom silicon.
 11. A system as claimed in claim 1, wherein the cavityextender is formed from silica.
 12. An optical coherence analysissystem, comprising: an interferometer for using an optical signal toanalyze a sample; and a tunable laser for generating the optical signal,the tunable laser including: a gain medium for amplifying opticalsignals within a laser cavity of the tunable laser; a tunable filter fortuning the optical signals over a spectral scan band and located in afree space portion of the laser cavity; and a cavity extender,comprising a transmissive substrate, in the free space portion of thelaser cavity between two intracavity reflectors to increase an opticaldistance between the two reflectors, the cavity extender being angledrelative to an optical axis of the tunable laser, wherein cavityextender is spaced away from the tunable filter.
 13. A tunable laser forgenerating frequency tunable optical signals, the tunable laserincluding: a gain medium for amplifying optical signals within a lasercavity of the tunable laser; a tunable filter for tuning the opticalsignals over a spectral scan band; and a cavity extender, comprising atransmissive substrate, in the optical cavity between two intracavityreflectors in a free space portion of the laser cavity to increase anoptical distance between the two reflectors, the cavity extender angledrelative to an optical axis of the tunable laser wherein cavity extenderis s aced away from the tunable filter.
 14. A method for producing atunable optical signal for an optical coherence analysis with reducedlaser pattern noise, the method comprising: generating the tunableoptical signal in a laser cavity that comprises a train of opticalelements including a gain medium, an intracavity tuning element, and atleast one lens component for coupling light between the gain medium andthe intracavity tuning element; directing the tunable optical signal toan optical coherence analysis system; and inserting a transmissive highrefractive index component in the laser cavity to increase an opticaldistance between optical elements that produce spurious reflections in afree space portion of the laser cavity, the transmissive high refractiveindex component being angled relative to an optical axis of the tunablelaser and being spaced away from the tunable filter.
 15. A method asclaimed in claim 14, wherein the step of inserting comprises insertingthe transmissive high refractive index component between the lenscomponent and the intracavity tuning element.
 16. A method as claimed inclaim 14, wherein the gain medium is a semiconductor gain medium.
 17. Amethod as claimed in claim 14, wherein the intracavity tuning elementcomprises a Fabry-Perot tunable filter.
 18. A method as claimed in claim14, further comprising antireflection coating the transmissive highrefractive index component.
 19. A method as claimed in claim 14, furthercomprising angling the transmissive high refractive index componentrelative to an optical axis of the laser cavity.
 20. A method as claimedin claim 14, wherein the high refractive index component is formed fromsilicon.
 21. A method as claimed in claim 14, wherein the highrefractive index component is formed from silica.
 22. An opticalcoherence analysis system, comprising: an interferometer for using anoptical signal to analyze a sample; and a tunable laser for generatingthe optical signal; the tunable laser including: a gain medium foramplifying the optical signal within a laser cavity of the tunablelaser; a tunable filter for tuning the optical signal over a spectralscan band and located in a free space portion of the laser cavity; alens component for coupling light between the gain medium and thetunable filter; a cavity extender, comprising a transmissive substratethat has both surfaces antireflection coated, in the free space portionof the optical cavity, the cavity extender being angled relative to anoptical axis of the tunable laser, wherein cavity extender is locatedbetween the lens component and the tunable filter and spaced away fromthe tunable filter to increase an optical distance between the lenscomponent and the tunable filter to change a position of the spuriousnoise peaks arising due to parasitic reflections between the lenscomponent and the tunable filter.